1. Field of the Invention
This invention relates to determining the attenuation coefficients within a body of material using sonic pulse techniques and more particularly to the use of sonic pulses to measure attenuation coefficients of internal tissue segments.
2. Description of the Prior Art
Sonic pulse techniques have been used to probe many varying types of material. For example, ultrasonic pressure waves have been used for over twenty years to probe internal tissues of the body in an attempt to diagnose pathological tissue states without invasive surgery. Low energy diagnostic ultrasound has gained popularity in the fields of neurology, ophthalmology, cardiology, obstectrics and gynecology. Most current methods using ultrasonic waves involve transmitting ultrasonic pulses into tissue segments and examining the characteristics of the returning echoes. In one known technique the amplitude of a returning echo is examined to provide an indication of the characteristics of the structure which causes the echo to be created. U.S. Pat. No. 4,058,001 illustrates an embodiment of this technique.
B- and M-scans, which are video displays, are used to generate darkened spots in the video display; the degree of darkness or "gray-scale" of such spots is related to the voltage amplitude of the echo. A paper authored by Gilbert B. Dewey and Peter N. T. Wells, entitled Ultrasound In Medical Diagnosis and published in Scientific American, Volume 238, May 1978, which paper is incorporated by reference herein provides further background in ultrasonic imaging techniques. Unfortunately, variations in amplitude of the returning echoes are related to many concurrent causes such as tissue structure, transducer center frequency and damping, absorption, singular echo duration, and time gain control and settings of the echo receiver. Therefore, it is always unclear whether the amplitude of a particular echo is due to an abnormality in the tissue segment or to one of the other causes set forth above. Interpretation of amplitude displays to determine structures other than simple tissue boundary locations requires a great deal of skill, sophistication, and standardization of echo receiver processing and time gain control settings. For a more detailed treatment of this type of interpretation see J. C. Birnholz, Visual Pattern Recognition and Clinical Ultrasonography, Second International Symposium on Ultrasonic Tissue Characterization, National Bureau of Standards, Session 7, June 7, 1977.
Currently, many investigators are attempting to determine the internal structure of tissue segments by measuring and displaying the physical properties of the segments including such parameters as attenuation coefficient, scattering coefficient, and boundary impedance difference. It is believed that such properties give a better indication of the structure of a tissue segment than the amplitudes of returning ultrasonic echoes. The leading method for generating and displaying these physical parameters is based on through-transmission tomographic reconstruction of ultrasonic wave propagation properties in which method the ultrasonic generating transducer and receiving transducer are separated by a fixed distance and the object to be examined is placed between them. However, through-transmission ultrasound can be used only for the examination of appendages such as breasts and testicles, and not for analysis of tissues deep within the pelvis and chest, because the amplitude of ultrasonic pulses which have passed through such areas are severely attenuated by bone and lung tissue and consequently are too weak to be analyzed with present instrumentation.
Attenuation of ultrasonic waves within tissue segments has received a great deal of attention over the last thirty years. Attenuation within a homogeneous tissue is described as the decrease in amplitude of the propagating ultrasonic wave due to conversion of acoustic energy to other forms of energy and due to scattering. Attenuation is defined as .alpha.(f,l)=log.sub.e A.sub.1 /A.sub.I where: .alpha.(f,l) is the attenuation and in general is a function of the propagation distance, l, and frequency, f; A.sub.1 is the amplitude of the propagating ultrasonic wave at a distance, l, from the generating transducer; and A.sub.I is the initial amplitude of the ultrasonic wave in the medium at the site of the generating transducer. In cases where .alpha.(f,l) are proportional to frequency, that is, .alpha.(f,l)=.alpha..sub.i fl, within a particular tissue, the proportionality constant, .alpha..sub.i, is defined as the attenuation coefficient. In a paper by D. E. Goldman and T. F. Hueter, J. Accoustical Society of America, 28:25, 1965, it is shown that attenuation is nearly proportional to ultrasonic frequency in the 1 to 10 MHz range for most tissues having a high protein content. Also, many investigators have shown a high correlation between attenuation in various types of tissue and the existence of abnormalities within that tissue. For a discussion in this subject, see P. P. Lele, et al., Tissue Characterization By Ultrasonic Frequency-Dependent Attenuation And Scattering, National Bureau of Standards Special Publication 453: 172, 1976; J. G. Miller, et al., Ultrasonic Attenuation In Normal And Ischemic Myocardium, National Bureau of Standards, Session 5, June 1977; M. O'Donnell, J. W. Mimbs, et al., Quantitative Collagen Concentration: A Determinate Of Attenuation In Myocardial Infarction, Proceedings AIUM, Sci. Session 2, Paper No. 1112, 1977; C. Calderon, D. Vilkomerson, R. Mezrich, et al., Differences In The Attenuation Of Ultrasound By Normal, Benign And Malignment Breast Tissue, Journal of Clinical Ultrasound, Volume 4, No. 4, p. 252, 1976. Therefore, if an accurate determination of a tissue segment's attenuation coefficient could be made, it could be reliably determined whether that tissue segment is normal or contains abnormalities.
Workers in the art have not been able to measure accurately tissue attenuation coefficients by pulse echo ultrasound. Measurement of peak echo amplitudes at two different ultrasonic center frequencies by using two different transducer crystals or narrow band pass filters, as in U.S. Pat. No. 4,057,049, often yields unreliable results because other leading and trailing echoes may interfere with the echo to be analyzed and because the measurements are taken during the nonsteady-state response of the instrumentation. For a discussion of this problem, see I. Beretski, et al., Impulse Response Detection In Pulse Echo Ultrasound Recent In Vitro Experiments With A Human Aorta, Second International Symposium On Ultrasonic Tissue Characterization, National Bureau of Standards, Session 7, June 1977.
Another unsuccessful technique for accurately measuring tissue attenuation coefficients involves computing Fourier power spectrums from time segments of echo wave trains. This method yields inaccurate results because the original time segments frequently contain echoes returning from adjoining tissue segments. For a further discussion of this technique, see L. Joynt, et al., Identification of Tissue Parameters By Digital Processing of Real Time Ultrasonic Clinical Data, Second International Symposium on Ultrasonic Tissue Characterization, National Bureau of Standards, Session 8, June 1977. In the Fourier computation the spectrums are computed from the summated effect of adjacent tissues instead of just one segment. Any attempt to reduce the number of echoes to one for purposes of spectral analysis leads to the taking of shorter and shorter data segments from the output of the echo receiver. However, the true spectrum of the desired echo is modified or blurred by shortening the length of the data segment. The extent of adjacent frequencies contributing to any one point in this modified spectrum increases with decreases in the duration of the echo. Thus, there is a trade-off between axial resolution and spectral resolution in time-gated pulse echo ultrasound technique.